It's just a matter of time before a huge earthquake hits the Pacific Northwest, experts say. Find out when it might happen.

Philippe Bourseiller/The Image Bank/Getty Images

When a magnitude-7.6 earthquake finally ripped open the offshore fault on Sept. 5, 2012, cracking buildings and collapsing a bridge, it was one of the first successful quake forecasts using GPS monitoring. (A forecast is a probability that an earthquake of a specific size will strike a specific location, and sometimes offers a time window. A prediction, on the other hand, says the quake will or will not occur during a specific time. It's like the difference between saying there's a 50 percent chance of rain on Monday and your house will be rained on Monday morning.)

"We're not predicting an earthquake," said Andrew Newman, a geophysicist at Georgia Tech. "The timing is something we still don't have a good handle on. But we can say where, and how big," Newman told LiveScience's OurAmazingPlanet.

Just a few months before the Costa Rica earthquake struck, an international team of scientists led by researchers in Costa Rica and the United States had forecast the size — a magnitude-7.8 — and location of the temblor. Their estimate was based on years of GPS measurements, which tracked subtle earth movements along the fault. Now, in a postquake analysis, the team has graded its forecasting performance. The results were published Dec. 22 in the journal Nature Geoscience.

"We now have the ability to very clearly identify reas that are locked up and building up energy for large earthquakes," said Newman, a study co-author.

The earthquake was centered about 25 miles (40 kilometers) below the Nicoya Peninsula, where million-dollar resorts draw ecotourists. Though buildings were damaged, no one was killed, in part due to the country's earthquake-conscious building codes. This gorgeous coastline also offers a unique dry lab for studying the shallow part of subduction zone earthquakes. (A subduction zone is the collision of two of Earth's tectonic plates. At the subduction zone, one plate dives deep into the Earth, bending beneath the other.) [Infographic: Tallest Mountain to Deepest Ocean Trench]

The thumb-like Nicoya Peninsula juts into the ocean, providing a platform for monitoring the shifting earth. At most subduction zones, miles of ocean lie between the shallow part of these faults and curious scientists. Geoscientists are interested in the shallow zone because this is where tsunamis are triggered. In Japan's 2011 Tohoku earthquake, the fault appeared to break all the way to the surface, surprising scientists — no subduction zone had cracked like this before. Big earthquakes in Sumatra and Costa Rica stopped deeper in the Earth.

"This is a really unique observatory for observing subduction zone environments," Newman said. "At this point, we still don't have a really good understanding of why the shallow zone locks or not."

With GPS and surveying by scientists, the team tracked where the Nicoya Peninsula bulged upward or sank downward — uplift or subsidence, in geology parlance. These surface changes revealed what has happening underground on the fault. The researchers were looking for zones where the fault locked, or stuck together.

"Just before the earthquake, there were places where the coconut groves [along the coast] were starting to be eroded away because relative sea level was getting higher and higher," Newman said. In those places, strain was building up in locked patches on the fault and pulling down the land, he said.